Activation of refractory metal filaments



Patented Jan. -15, 1929.

. UNITEDSTA Joan wnsnnx mnnmor EAST m2 comm,-va

ORANGE, NEW JERSEY, ASSIGNOR TO WESTINGHOUSE CORPORATION OFPENNSYLVANIA.

nc'rrvarroiw or nnrnac'ronx METAL FILAMENTS.

No Drawing.

This invention relates to the activation of material forelectron-emission.purposes and more particularly to the useof ahalide ofthorium or the like and aluminum'for such 6 purposes.

An object of my invention is the preparation of filamentary materialcoated with metallic thorium, whereby good electron emissivity isimparted thereto. 7

Another object of my invention is the'application of a mixture of halideof thorium and finely powdered aluminum, whereby when the same isheated, metallic tho: rium is developed thereon to give good activation.

A further object of my invention is the use of thorium fluoride or ananhydrous double fluoride of thorium and an alkali metal,

mixed with powdered aluminum, for activa- 2 tion purposes, either byapplying the mixture directly to a filament or cathode forelectron-emission or by application to a component p ple, the plate oranode of a radio tube and transfer of thorium developed thereon by heat,to an adjacent filament.

A still further object of my invention is the utilization of a halide ofa refractory metal, such as thorium, uranium, zirconium or the like,preferably the fluoride or double fluoride with an alkali metal, mixedwith powdcred aluminum for-the preparation of the respective refractorymetal for activation or other purposes.

Other objects'andtadvantages of the in-v vention will be made apparentupon reading the following description.

Various methods have been employed for developing thorium on filamentarymaterial in evacuated vessels, for electron-emission purposes. a I p Inthe copending application of C. T. Ulrey, Serial No. 577,021, filed July24'. 1922, manufacture of electron-emitting devices and inghouse LampCompany, is. disclosed a method of applying thorium to the eectronemitting filament in a radio tube or the like by applying a. ieceof metallic thorium 'tothe plate thereo applying finely divided thoriumart of an electron device, for exam-.

' he-likeand assigned to the Westthe plate, a mixture of thoria,

be emp hpplieationflled Harch 31, '1923. Serial No.629,190. I

in the tube by highly heating the plate in any by making desired manner,for example, by electron,

bombardment or high freqHency induction, to cause a transfer of thoriumfrom the plate to the filament.

' In my copending application Serial'No. 629,189, filed March 31, 1923,electron devices and their manufacture, and assigned to the\Vestinghouse Lamp Company, is disclosed a method of activation, similarto that of the Ulrey application above referred to,

except that, instead of using free thorium on or other thorium compound,and carbon may be applied thereon,'with or without a clean-up agent,such as aluminum or magnesium. :Such a mixture on the plate issubsequently heated to cause a. liberation of thorium and transferthereof, at least in part, to the electron-emitting filament. I

-Both of the aforementioned methods are desirable but the former entailsthe use of metallic thorium which is expensive and difficult' to keep inan unoxidized or uncontaminated condition. The latter obviates the useof expensive metallic thorium but entails the use of such material ascarbon which results the liberation of gaseous carbon compounds whichare undesirable as part of the residual atmosphere in the bulb andnecessitate an extra quantity of clean-up materlal "for theirelimination. v According to my present invention, I

avoid the use of expensive metallic thorium and the generation ofundesirable gaseous carbon compounds by using a salt of thorium which isreducible by aluminum'or other desirable metal. The aluminum may servethe double purpose of reducing the thorium compound used and cleaning upresidual gases in the bulb.

' It has been found that the class of salts of thorium most suitable forthe purpose are the halidesyof which the fluoride is preferred.

Although thorium tetra-fluoride: (ThFg may oyed, for certain reasons aouble fluoride of thorium and an alkali metal, for example,potassium-thorium fluoride, having the formula KThF or K ThF ispreferred. Such material, preferably mixed with an excess of finelydivided aluminum, may be applied or gettered on a tungsten wire, forexample, in any desired manner, using cryolite mixed therewith as abinder, if desired, or painted on or attached to the plate for a radiotube and sealed into a bulb. After exhausting the bulb, the wire orfilamentary cathode therein may be activated by heating the same, if themixture was applied directly thereto, or if the mixture was merelyapplied to the plate adjacent thereto, by heating said plate untilthorium is liberated therefrom and transferred to the filament.

In the following description, my invention will be explained more indetail.

It is known in metallurgy that thorium chloride, for example, may bereduced by sodium but neither thorium chloride nor sodium can be exposedto the air without con-- tamination, nor can they be gettered or appliedto a wire or filament in any known manner without complete oxidation ofboth the sodium and the thorium chloride.

In my copending' application Serial No. 498,397, filed September 3,1921, the preparation of metals and their halides and assigned to theWestinghouse Lamp Company, is disclosed a method for the production ofthe anhydrous halides of refractory metals, includin" thorium, by thetreatment of a hydrous salt of such a metal with an agent, for example,ammonium chloride, which will displace the water from such salt. Suchmethod is particularly adapted for producing thorium chloride, which maythen be used for the production of other halides of thorium, moreparticularly adaptable for practicing the present invention. Forexample, thorium chloride, produced in any desired manner or accordingto the method disclosed in my referred-to'application, may be fused witha small excess of potassium fluoride to produce potassium-thoriumfluoride (KThF or K,ThF according to one of the following equations Theexact proportions in which these salts are fused, is not essential, as asmall excess of potassium fluoride makes no difference. After completefusion, the mass is cooled powdered and the soluble constituents thereofdissolved i'iutby water. The excess of potassiuni fluoride'dissolvesaway, leaving the 'dou'- ble thorium salt desired, which'is' insolublein water and dilute acids. This should then be ground, as finel; aspossible, and mixed with aluminum, which is preferably in a finelydivided condition and in excessof that 1 ne d to cmnuletelv reduce thethorium C ntained in the compound according to the fol lowing equation Vrectly. This salt may be prepared by -pre cipitation from a watersolution of the chloride, but is not always free from oxide when made inthis way. It is, however, suitable for activation purposes.

The double halide of thorium and an alkali metal or the halide ofthorium which, as heretofore mentioned, it is desired to use, may thenbe mixed in the form of a dry powder with finely divided aluminum andapplied either to a filament or wire to be activated or to a-plateintended to be used in cooperation therewith. The proportion of aluminumand the halide salt used, does not hate to be exact, but it ispreferable that there shall be an excess of aluminum over that needed toreduce the thorium in said salt to the metal, for the purpose ofcleaning up residual gases in the bulb, although, of

course, if such excess is not employed, some other clean-up agent, forexample, magnesium, may be used instead.

The mixture of potassium-thorium fluo ride and aluminum, for example,may then be applied to a filament or wire of'tungsten or the like, whichit is desired to use for electron-emitting purposes, either as anaqueous suspension or paste or in the form of a paint with some suitablevehicle or hinder, such as an amyl acetate solution of nitrocellulose orwith cryolite, in a manner similar to that, for holding thoria and thelike ona filament, and described and claimed in the copendin applicationof MacRae and Richardson, Se 8, 1922, getters and the applicationthereof and assigned to the Westmghouse Lamp Go. The filament may thenbe sealed into a bulb and heated to incandescence until the thorium isreduced, as may be determined by noting the increase in the thermioniccurrent therefrom, in any desired manner, or in the manner disclosed inthe application of C. 'T. Ulrey, before referred to. v

Instead of applying the mixture directly to the filament, the same maybe applied instead, to a plate or anode intended to be used in the bulbin cooperation with, and adjacent to said filament. 'After sealing boththe filament and the coated plate into a bulb and evacuatin said bulb,the filament may be activated by heating the plate in any desiredmanner, for example, by electron bombardment orhizzh-freguency indueiiomuntil the rial No. 566,847, filed Junev IUD - asaforedescribed, the

filament becomes activated to a suitable degree by a transfer ofliberated thorium thereto from the plate in a manner similar to thatdisclosed in the-aforementioned Ulrey application.

The excess of aluminum serves .to clean up residual gases in the bulb,making it possible to use a cold exhaust as disclosed in the co-'gending application of H. C. Rentschler, erial No. 582,261, filed August:16, 1922, vacuum devices-and method of exhausting the same and assignedto the Westinghouse Lamp Company.

It is obvious closure of means into a vacuum device, device containingan inert gas pressure, sired purpose. For example,- if it is desired tointroduce calcium or any element, for that matter, a mixture of acompound or salt thereof with a reducing agent, such as aluminum, may beapplied to the plate'of a for introducing a metal or even an evacuatedat reduced ample, or even upon an auxiliary plate m a lamp. The mixtureis subsequently heated,

and the desired metal, for example, is liberated for accomplishing thepurpose, that be the activation of an electron-emitting filament, thecleaning-up of residual gases for combination with which'it isparticularly adapted, or the purification of an inert gas filling.

Although thorium has been-mentioned as;

the preferred activatin material, electronemitting filaments may %eactivated by other metals, having good electron-emission properties, forexample, uranium, zirconium and the like, in the manner disclosed foractivating by means of thorium and Ido not wish to be restrictedentirely to thorium, but

desire that it be' understood that such modifications as fall within thescope of the appended claims pertain to my mvention. What is claimed is:v 1. The method of activating, in a. vacuum,

a refractory filament for electron-emission purposes comprising placingadjacent thereto, a mixture of a halide of thorium and powdered aluminumand heating said mixture to cause metallic'thorium to be liberated andform on 'the filament.

2. The method of emissivity ofa filamentary comprising placing adj aceture of a double halide alkali metal with finely divided aluminum andheating said mixture to cause a reduction of said thorium compound and aformation of thorium on the cathode.

3. The method of activating a refractory increasing the electronmetalcathode thereto, a mix.-

filament for electron-emission purposes comprising applying thereto, amixture of a halide of a metal of good electron-emission metal on thefilament.

that the foregoing'is a disfor activation or for any 0 her deblndlngmaterial, enclosing the same n an thorlum, uranlum, zirconium,

radio tube, for .ex-

compound is reduced whether ated envelope into double function ofthorium and an properties and powdered aluminum, enclosing the same inan evacuated vessel and heating the filament to cause a reaction in theapplied mixture with a liberation of the free 4. The method ofactivating a refractory metal filament for electron-emission purposes,comprising applying thereto, a mixture of a fluoride of thoriumand-aluminum with "a suitable binding material, enclosing the same inan" evacuated envelope and heating the filament to cause areduction andliberation of free "metallic thorium thereon.

5. The method of activating a refractory metal filament forelectron-emission purposes comprising applying thereto, a mixture ofp'otassiumthorium fluoride and powdered aluminumheld thereon by means ofa suitable evacuated envelope and heating the filament to cause aninteraction between the thorium compound and the aluminum with theliberation of free metallic thorium on the filament.

.6.- An electron device comprising an evacuated vessel. having sealedtherein a cathode, an anode and a mixture of a halide of thorium andaluminum applied to at least one of said eleme ts.

n electron devlce comprising an evacuwhich is sealed a filament of whichhas been applied a metalsuitable for a metallic and a plate, to one amixture of a halide of electron-emission purposes, and. reducing agent.

8. An electron ated-envelope enclosing a cooperating therewith andad'acent. thereto and a mixture aluminum applied to one of saidelements. 9. An electron device comprising an evacudevice comprising anevacuated vessel enclosing an electron-emitting cathode and acooperating anode, one of said elements having applied thereto, amixture of potassium-thorium fluoride and anexcess of aluminum, wherebysaid aluminum serves the of reducing the thorium'salt to liberatethorium and to clean. up residual gases in the vessel. 4

10. An electrondevice comprising an evacuated'envelope into wh1ch issealed a cathode and an anode, and a mixture ofa fluoride .of j

'1 filament and a plate of a fluoride o thorium 'and' thorium andpowdered aluminum applied to saidcathode." u

11. An electron device.'comprising an evacuated envelope enclosing afilamentary cathode coated with .a mixture of potassiumthoriu'm'fluoride and an excess of aluminum, and an anode cooperating therewith.

'12. An electron device comprising an evacuated vessel enclosing afilament and a cooperating electrode coated with a mixture of thoriumfluoride and aluminum and held thelreon by means of a suitable bindingmate r1a 13. The-method of introducing thorium into an evacuated devicecomprising mixing potassium thorium fluoride with powdered aluminum,applying said mixture to an element in the device and heating to cause areaction With the liberation of free thorium therein.

14. An evacuated device comprising a vessel enclosing a mixture ofpotassium thorium fluoride and aluminum, said mixture being applied on aconductiveelement in the device whereby the same may be heated to effecta reaction in the mixture.

In testimony whereof, I have hereunto subscribed my name this 30th dayof March, 1923.

JOHN WESLEY MARDENI

